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Diffstat (limited to 'numpy/lib/shape_base.py')
| -rw-r--r-- | numpy/lib/shape_base.py | 227 |
1 files changed, 226 insertions, 1 deletions
diff --git a/numpy/lib/shape_base.py b/numpy/lib/shape_base.py index 41ef28ef3..65104115a 100644 --- a/numpy/lib/shape_base.py +++ b/numpy/lib/shape_base.py @@ -16,10 +16,235 @@ from numpy.matrixlib.defmatrix import matrix # this raises all the right alarm __all__ = [ 'column_stack', 'row_stack', 'dstack', 'array_split', 'split', 'hsplit', 'vsplit', 'dsplit', 'apply_over_axes', 'expand_dims', - 'apply_along_axis', 'kron', 'tile', 'get_array_wrap' + 'apply_along_axis', 'kron', 'tile', 'get_array_wrap', 'take_along_axis', + 'put_along_axis' ] +def _make_along_axis_idx(arr_shape, indices, axis): + # compute dimensions to iterate over + if not _nx.issubdtype(indices.dtype, _nx.integer): + raise IndexError('`indices` must be an integer array') + if len(arr_shape) != indices.ndim: + raise ValueError( + "`indices` and `arr` must have the same number of dimensions") + shape_ones = (1,) * indices.ndim + dest_dims = list(range(axis)) + [None] + list(range(axis+1, indices.ndim)) + + # build a fancy index, consisting of orthogonal aranges, with the + # requested index inserted at the right location + fancy_index = [] + for dim, n in zip(dest_dims, arr_shape): + if dim is None: + fancy_index.append(indices) + else: + ind_shape = shape_ones[:dim] + (-1,) + shape_ones[dim+1:] + fancy_index.append(_nx.arange(n).reshape(ind_shape)) + + return tuple(fancy_index) + + +def take_along_axis(arr, indices, axis): + """ + Take values from the input array by matching 1d index and data slices. + + This iterates over matching 1d slices oriented along the specified axis in + the index and data arrays, and uses the former to look up values in the + latter. These slices can be different lengths. + + Functions returning an index along an axis, like `argsort` and + `argpartition`, produce suitable indices for this function. + + .. versionadded:: 1.15.0 + + Parameters + ---------- + arr: ndarray (Ni..., M, Nk...) + Source array + indices: ndarray (Ni..., J, Nk...) + Indices to take along each 1d slice of `arr`. This must match the + dimension of arr, but dimensions Ni and Nj only need to broadcast + against `arr`. + axis: int + The axis to take 1d slices along. If axis is None, the input array is + treated as if it had first been flattened to 1d, for consistency with + `sort` and `argsort`. + + Returns + ------- + out: ndarray (Ni..., J, Nk...) + The indexed result. + + Notes + ----- + This is equivalent to (but faster than) the following use of `ndindex` and + `s_`, which sets each of ``ii`` and ``kk`` to a tuple of indices:: + + Ni, M, Nk = a.shape[:axis], a.shape[axis], a.shape[axis+1:] + J = indices.shape[axis] # Need not equal M + out = np.empty(Nk + (J,) + Nk) + + for ii in ndindex(Ni): + for kk in ndindex(Nk): + a_1d = a [ii + s_[:,] + kk] + indices_1d = indices[ii + s_[:,] + kk] + out_1d = out [ii + s_[:,] + kk] + for j in range(J): + out_1d[j] = a_1d[indices_1d[j]] + + Equivalently, eliminating the inner loop, the last two lines would be:: + + out_1d[:] = a_1d[indices_1d] + + See Also + -------- + take : Take along an axis, using the same indices for every 1d slice + put_along_axis : + Put values into the destination array by matching 1d index and data slices + + Examples + -------- + + For this sample array + + >>> a = np.array([[10, 30, 20], [60, 40, 50]]) + + We can sort either by using sort directly, or argsort and this function + + >>> np.sort(a, axis=1) + array([[10, 20, 30], + [40, 50, 60]]) + >>> ai = np.argsort(a, axis=1); ai + array([[0, 2, 1], + [1, 2, 0]], dtype=int64) + >>> np.take_along_axis(a, ai, axis=1) + array([[10, 20, 30], + [40, 50, 60]]) + + The same works for max and min, if you expand the dimensions: + + >>> np.expand_dims(np.max(a, axis=1), axis=1) + array([[30], + [60]]) + >>> ai = np.expand_dims(np.argmax(a, axis=1), axis=1) + >>> ai + array([[1], + [0], dtype=int64) + >>> np.take_along_axis(a, ai, axis=1) + array([[30], + [60]]) + + If we want to get the max and min at the same time, we can stack the + indices first + + >>> ai_min = np.expand_dims(np.argmin(a, axis=1), axis=1) + >>> ai_max = np.expand_dims(np.argmax(a, axis=1), axis=1) + >>> ai = np.concatenate([ai_min, ai_max], axis=axis) + >> ai + array([[0, 1], + [1, 0]], dtype=int64) + >>> np.take_along_axis(a, ai, axis=1) + array([[10, 30], + [40, 60]]) + """ + # normalize inputs + if axis is None: + arr = arr.flat + arr_shape = (len(arr),) # flatiter has no .shape + axis = 0 + else: + axis = normalize_axis_index(axis, arr.ndim) + arr_shape = arr.shape + + # use the fancy index + return arr[_make_along_axis_idx(arr_shape, indices, axis)] + + +def put_along_axis(arr, indices, values, axis): + """ + Put values into the destination array by matching 1d index and data slices. + + This iterates over matching 1d slices oriented along the specified axis in + the index and data arrays, and uses the former to place values into the + latter. These slices can be different lengths. + + Functions returning an index along an axis, like `argsort` and + `argpartition`, produce suitable indices for this function. + + .. versionadded:: 1.15.0 + + Parameters + ---------- + arr: ndarray (Ni..., M, Nk...) + Destination array. + indices: ndarray (Ni..., J, Nk...) + Indices to change along each 1d slice of `arr`. This must match the + dimension of arr, but dimensions in Ni and Nj may be 1 to broadcast + against `arr`. + values: array_like (Ni..., J, Nk...) + values to insert at those indices. Its shape and dimension are + broadcast to match that of `indices`. + axis: int + The axis to take 1d slices along. If axis is None, the destination + array is treated as if a flattened 1d view had been created of it. + + Notes + ----- + This is equivalent to (but faster than) the following use of `ndindex` and + `s_`, which sets each of ``ii`` and ``kk`` to a tuple of indices:: + + Ni, M, Nk = a.shape[:axis], a.shape[axis], a.shape[axis+1:] + J = indices.shape[axis] # Need not equal M + + for ii in ndindex(Ni): + for kk in ndindex(Nk): + a_1d = a [ii + s_[:,] + kk] + indices_1d = indices[ii + s_[:,] + kk] + values_1d = values [ii + s_[:,] + kk] + for j in range(J): + a_1d[indices_1d[j]] = values_1d[j] + + Equivalently, eliminating the inner loop, the last two lines would be:: + + a_1d[indices_1d] = values_1d + + See Also + -------- + take_along_axis : + Take values from the input array by matching 1d index and data slices + + Examples + -------- + + For this sample array + + >>> a = np.array([[10, 30, 20], [60, 40, 50]]) + + We can replace the maximum values with: + + >>> ai = np.expand_dims(np.argmax(a, axis=1), axis=1) + >>> ai + array([[1], + [0]], dtype=int64) + >>> np.put_along_axis(a, ai, 99, axis=1) + >>> a + array([[10, 99, 20], + [99, 40, 50]]) + + """ + # normalize inputs + if axis is None: + arr = arr.flat + axis = 0 + arr_shape = (len(arr),) # flatiter has no .shape + else: + axis = normalize_axis_index(axis, arr.ndim) + arr_shape = arr.shape + + # use the fancy index + arr[_make_along_axis_idx(arr_shape, indices, axis)] = values + + def apply_along_axis(func1d, axis, arr, *args, **kwargs): """ Apply a function to 1-D slices along the given axis. |